Dython

* Added new functionality to `model_utils.roc_graph` (Plot best threshold, print class names)

* Added `nominal.cluster_correlations`, and an option to cluster `nominal.associations` heatmap (by **[benman1](https://github.com/benman1)**)

* Added automatic recognition of categorical columns in `nominal.associations` (by **[benman1](https://github.com/benman1)**)

* `nominal.associations` can accept an exisiting Matplotlib `Axe` (issue [24](https://github.com/shakedzy/dython/issues/24), by **[Baukebrenninkmeijer](https://github.com/Baukebrenninkmeijer)**)

* Introducing missing values handeling (`nan_strategy`) in `nominal` module (issue [15](https://github.com/shakedzy/dython/issues/15))

* Added `sampling` module